D-PE, for which demand is increasing as a starting material for, e.g., polyester, polyurethane, thermal stabilizers for polyvinyl chloride resins or lubricating oils, is a by-product obtained when PE is synthesized i.e., when PE is produced by reacting formaldehyde and acetaldehyde in the presence of an alkali. In conventional processes commonly used, this by-product is separated and purified to obtain the D-PE. In such processes, the following methods have been proposed in order to increase the rate of secondary formation of D-PE.
(1) A method in which the molar ratio of formaldehyde to acetaldehyde charged in a reaction system for the synthesis of PE is controlled to be smaller than 4, the theoretical molar ratio;
(2) a method in which the reaction is carried out using formaldehyde, acetaldehyde and an alkali agent in high concentrations; and
(3) a method in which the reaction is carried out by adding and dissolving PE in an aqueous formaldehyde solution.
These methods, however, have had the disadvantages that PE and D-PE can be obtained in a low total yield and the main product PE has a poor quality. As an improved method of these methods:
(4) a method has been proposed in which the reaction is carried out by previously charging some amounts of formaldehyde, alkali and acetaldehyde in a reaction vessel and then simultaneously dropwise adding formaldehyde, alkali and acetaldehyde in amounts not less than their theoretical molar amounts while keeping reaction temperature at 50.degree. C. or less (Japanese Patent Publication No. 1-44689).
Also known is (5) a method in which PE is subjected to dehydration condensation using phosphoric acid, sulfuric acid or the like to synthesize polypentaerythritol (U.S. Pat. No. 2,402,047).
Besides, also proposed as methods of separating D-PE from a mixture of PE and D-PE are;
(6) a method in which D-PE is crystallized from a D-PE/PE mixed solution prepared in a proportion of 30/70 or more (U.S. Pat. No. 2,448,566; and
(7) a method in which D-PE is crystallized from a mixed solution having a PE/D-PE weight ratio of not more than 16 and a specific PE concentration (Japanese Patent Publication No. 2-10811).
However, the conventional method of producing D-PE by reacting acetaldehyde and formaldehyde [the above method (4)] has been involved in the problem that the purification step must be complicated in order to separate from PE and D-PE, by-product impurities such as sodium formate, bispentaerythritol monoformal (hereinafter "B-PE") and excess formaldehyde or an acetaldehyde-formaldehyde self-condensation product. It also has the problem that the production of D-PE can not overtake the increase in recent demand since the production possible for D-PE depends on the production of PE and is limited to 10 to 15% of the production of PE.
The method in which a polypentaerythritol mixture is synthesized from PE using phosphoric acid, sulfuric acid, an aromatic sulfonic acid or the like [the above (5)] aims at synthesis of various polypentaerythritol mixtures and hence does not clearly teach any means for selectively synthesizing D-PE. According to a finding of the present inventors, D-PE is firstly formed when PE is subjected to dehydration condensation using an acid catalyst, but the D-PE thus formed successively changes into tripentaerythritol (hereinafter "T-PE") and then into a high-molecular weight polypentaerythritol or intramolecular condensation product. Thus, the present inventors have found that this method is not practical as a process for producing D-PE.
The method disclosed in U.S. Pat. No. 2,448,566 [the above (6)] discloses a technique by which D-PE is separated from a mixture of PE and D-PE, and does not disclose the method of synthesizing D-PE in which the PE is subjected to self-condensation and the D-PE is obtained from the resulting reaction mixture. Accordingly, the method disclosed in this U.S. Patent has never been a method of producing D-PE independently of the manufacture of PE.
The method described in Japanese Patent Publication No. 2-10811 [the above (7)] has had, in addition to the same problems as in the above methods (1) to (4), the problem that it requires a troublesome step of adjusting PE concentration to a given range.
Objects of the present invention are to solve the above problems, and to provide a process of producing D-PE that can produce D-PE in a good efficiency, furthermore gives impurities in a low concentration, enables easy purification, and can obtain a high-quality product.